With the increasing use of satellites for business data communications there is a growing need for data modems with good efficiency of bandwidth and power.
Recent developments in high efficiency modems have tended to concentrate on power efficiency. Thus a modern data modem may take bits one or two at a time from a data stream that has been previously subject to Forward Error Control, and use these bits to control the level of phase modulation of a carrier during a corresponding modulation time interval, the resultant transmitted symbols each having two or four possible levels (this modulation process is generally referred to as two-or four-level phase shift keying, PSK). These modern modems have brought down the required satellite power by a factor in excess of 4 to 1 (6dB) with respect to the requirements of only a few years ago. However, this improved power efficiency has been at the expense of occupied bandwidth, which, allowing for adjacent channel guardbands, can be up to 3 or 4 times the data rate. At lower data rates there is sufficient bandwidth available for this to be accommodated but in the 2 or 4 Mbit/sec data rate range, bandwidth efficiency becomes much more critical.
One method of improving bandwidth efficiency is to increase the number of possible levels of each symbol to increase the number of transmitted bits/Hz of occupied bandwidth. However as the number of levels per symbol is increased, the symbol power has to be increased dramatically to prevent noise from increasing the bit error rate. The need for this dramatic increase is due to the fact that as the numbers of levels per symbol is increased, the different symbol combinations are not orthogonal to each other as in the two or four level symbol case and there is degradation similar in effect to cross-talk.
It is an object of the present invention to provide a data communication method and apparatus with good bandwidth and power efficiency.